The present invention relates to a ferroally for the treatment by inoculation of cast iron with spheroidal graphite having to be used as cast, as well as to a process for treating the liquid cast iron with this ferroalloy.
Well known procedures for treating liquid cast iron in general which are carried out in the following order: carburization, desulfurization, spheroidizing, inoculation, most often comprise a post-inoculation treatment carried out with inserts of ferroalloy introduced into the molds to refine the structure by obviating the imperfections of the above-mentioned treatments carried out upstream.
The variable number of spheroids produced in moulding has different models of specifications; indeed the ferrite/pearlite ratio of the matrix is largely dependent on the structure of the graphite in cast iron of current chemical composition. It so happens that, for these cast iron, the desired morphology of the graphite is generally obtained directly by the spheroidizing treatment, either by an addition of magnesium or by an addition of ferroalloy containing magnesium. The quantity of spheroidizing ferroalloy is determined by well-known factors such as the sulfur contained in the base iron.
This constraint which determines the overall quantity of ferroalloy for treatment cannot enable the optimum quantity of each element constituting the alloy to be distinguished.
To complete the desired graphitic presentation, rare earth metals are then used which, in well metered quantities, have a particularly favourable and well known effect. Thus, in the majority of cases, the addition of rare earth metals to the cast iron is necessary to neutralize the contaminating elements which may be brought by the base materials. However, excessive addition of rare earths can produce mottled structures due for example to the carbides, by reason of the behaviour of the rare earth metals in the cast iron. They may also degenerate the spheroids of graphite and/or reduce the quantity thereof.
Whether the rare earth metals are a constituent part of the magnesium-based ferroalloys or whether they are incorporated directly in the liquid cast iron the dosages remain very tricky. This results in an often fluctuating yield which sometimes leads to the necessity of resorting to the use of inserts in order to post-inoculate in the moulds, and sometimes to the undesirable, detrimental appearance of forms of degenerated graphite in the solidified structure.
Furthermore, the action of bismuth, lead or antimony, as far as neutralizing the spheroidizing action is concerned, is well known. The increasing presence of these elements in the cast metal thus leads to the appearance of structures of degenerated graphite which, in the event of overdose, cannot always be prevented by the addition of rare earth metals. Thus, bismuth, lead or antimony may sometimes cause a considerable increase in the number of spheroids, but the capacity of these elements for degenerating the spheroidal structure of the graphite has, up to the present time, prevented general application thereof.